The US Standard railroad gauge (distance between the rails) is 4 feet,
8.5 inches. That's an exceedingly odd number. Why was that gauge used?
Because that's the way they built them in England, and the US
railroads were built by English expatriates.

Why did the English people build them like that? Because the first
rail lines were built by the same people who built the pre-railroad
tramways, and that's the gauge they used.

Why did "they" use that gauge then? Because the people who built the
tramways used the same jigs and tools that they used for building
wagons, which used that wheel spacing.

Okay! Why did the wagons use that odd wheel spacing? Well, if they
tried to use any other spacing the wagons would break on some of the
old, long distance roads, because that's the spacing of the old wheel
ruts.

So who built these old rutted roads? The first long distance roads in
Europe were built by Imperial Rome for the benefit of their legions.
The roads have been used ever since. And the ruts? The initial ruts,
which everyone else had to match for fear of destroying their wagons,
were first made by Roman war chariots. Since the chariots were made
for or by Imperial Rome they were all alike in the matter of wheel
spacing.

Thus, we have the answer to the original questions. The United State
standard railroad gauge of 4 feet, 8.5 inches derives from the
original specification (Military Spec) for an Imperial Roman army war
chariot. MilSpecs and Bureaucracies live forever.

So, the next time you are handed a specification and wonder what
horse's ass came up with it, you may be exactly right. Because the
Imperial Roman chariots were made to be just wide enough to
accommodate the back-ends of two war horses.